1. Field of the Invention
The invention relates to an apparatus and method for the utilization of the heat energy normally discarded in the exhaust of internal combustion engines (or from other sources) by converting the heat to mechanical work in a highly efficient manner either using the phase change properties of a working medium or heating a fluid that is pressurized above its critical pressure, thereby increasing the overall efficiency of the engine. The field of application is primarily in internal combustion engines for motor vehicles.
2. Prior Art
The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including oxides of nitrogen and greenhouse gases such as carbon dioxide. Accordingly, the need exists for innovations to significantly improve the efficiency of fuel utilization for automotive powertrains.
Internal combustion engines create mechanical work from fuel energy by combusting the fuel over a thermodynamic cycle consisting typically of compression, ignition, expansion, and exhaust. Expansion is the process in which high pressures created by combustion are deployed against a piston, converting part of the released fuel energy to mechanical work. The efficiency of this process is determined in part by the thermodynamic efficiency of the cycle which, in turn, is determined in part by the final pressure and temperature to which the combusted mixture can be expanded while performing work on the moving piston. Generally speaking, the lower the pressure and temperature reached at the end of the expansion stroke, the greater the amount of work that has been extracted.
Conceptually, the work that is performed on the piston can be resolved into two components. One component is the fuel energy released by the combustion process. Another is the compression energy that is returned as the compressed mixture expands again after piston top dead center (TDC) as it naturally would, with or without combustion. When fuel is injected in a liquid state, a phase change (vaporization) occurs which consumes some of the energy present in the compressed mixture that now cannot be reclaimed in the expansion. After combustion has been initiated and expansion of the combustion products begins, the amount of energy in the cylinder available to be delivered via expansion is fixed. At this point all that remains is to expand the high temperature and pressure combustion gases to as near ambient conditions as possible considering the engine design and the properties of the combustion products. Expansion is limited by the fixed maximum volume of the cylinder, since there is only a finite volume available in which combusting gases may expand and still be performing work on the piston. Even if expansion to atmospheric pressure is achieved, the expanded gases are still at high temperature, often exceeding 1000.degree. F. Thus, not only is potential work lost to vaporization of the fuel, but a large amount of potential work is lost as exhaust heat.
A prior art means of utilizing exhaust heat by way of heat transfer to and phase change of a working fluid in a separate system is the well known Rankine Bottoming Cycle. Water is most often used followed by condensation and recycle of the water. Other closed system working fluids may also be used. However, all such systems are costly and have relatively low energy recovery efficiency primarily because much of the energy from the exhaust gas is consumed by the phase change (evaporation) of the working fluid and this energy is mostly lost again in the condenser.